After drilling a wellbore in the oil and gas industry, the drilled wellbore can be subsequently completed by cementing a string of metal pipes connected end-to-end within the wellbore. Commonly called “casing,” such pipes increase the integrity of the wellbore and provide a flow path between the earth's surface and selected subterranean formations. Some wellbores are lined with multiple concentrically-positioned pipes (i.e., concentric strings of casing). Moreover, in some wellbores, one or more production pipes are extended into a cased wellbore to provide a conduit for hydrocarbons to be conveyed to the earth's surface. Accordingly, as used herein, the term “pipe” or “wellbore pipe” will refer to metal pipes or pipelines that line the walls of a wellbore, such as casing, and may also refer to production pipes extended into a wellbore to facilitate hydrocarbon production operations.
During the lifetime of a well, wellbore pipes are exposed to high volumes of materials and fluids required to pass through them, including chemically aggressive fluids. In harsh environments, the pipes may be subject to corrosion that may affect their functionality. Consequently, the structural integrity of wellbore pipes may change over time due to chemical and mechanical interactions. Moreover, due to the length, volume, accessibility difficulties, and long time periods associated with the process, it is a costly task to monitor wellbore pipes and pipelines and intervene when required.
Electromagnetic (EM) sensing technologies and techniques have been developed for such monitoring applications and can generally be categorized into two groups: frequency-domain techniques and time-domain techniques. In frequency-domain techniques, measurements of the wellbore pipes are typically performed at a high frequency to characterize the innermost pipes and at a low frequency to characterize the outermost wellbore pipes. Time-domain techniques are based on the pulse eddy current and employ the transient response (decay response versus time) of the pipes to a pulse excitation. Proper analysis of the signal responses can determine metal losses in the pipes with better resolutions, and also improve the robustness of the characterization process to noise.
While EM sensing provides continuous, in situ measurements of the integrity of wellbore pipes, the available EM inspection tools do not effectively facilitate evaluation of multiple concentrically-positioned wellbore pipes.